How do I get my extreme deviation or spread DOWN?

[Lazerus]

Thanks for al lthe responses on my last post. I have a newly built 6.5 284 Norma built on a tika action with a 28 inch Burx barrel giving me some nice little 1/4 clusters at 100 yards. I have tried two powders, reloader 22 and H4831SC. Everybody says to go with Hodgeson so that is what I am going to do. So here is what my velocities are now:
2980
2957
2980
2985
2963
2981
2964
Basically I only hae a 100 yard range to work with and I only have CCI bench rest primers. I am shooting Berger 140 grain VLD's at 10.000ths down from full jam. I could probably move the bullet in alittle bit, but the group starts to open up if I move it out. I am just not sure what the practices are to get this extreme spread down. ANy advice would be greatly helpful...forgot to mention I am loading 52.8 grains of Hodgeson.

 

[jkupper]

Have you been using an electronic scale to measure your charge for each shot, or have you been hand trickling to get a precise measure? That may make a difference. Also wondering if you have weight sorted your brass? Best of luck! Sounds like you have a good shooting gun anyway

 

[MMERSS]

What were your weather conditions? cloudy, sunny, changing etc.
What chronograph are you using?

It's not uncommon to have a fluctuation in MV readings for some chronographs with a change in lighting conditions plus their inherent error during readings. My Chrony chronographs I only trust so much. With a Chrony, if I'm around the 20 to 30 FPS ES and good 100 yard results I'm off to test at least 300 yards and verify my vertical spread. Oehler chronographs are significantly more reliable, predictable and accurate.

If you are certain of your MV readings a slight change in bullet seating depth and powder charge weight can change the ES, for the better or worse. Making slight adjustments in one direction or the other can make all the difference.

 

[Lazerus]

I am weighing and sorting my cases by weight. I am turning necks, I am trickling my powder in to get the most accurate powder charge I can. I am basically taking every precaution I can to make the most accurate ammo I can. It is interesting you ask about lighting, because on the day I chronographed it was kind of hazy/foggy with intermittent blue sky. I don't even know if this could make a difference. I have tight match chamber and when I use my body die to re-size the bodies, most of the time it leaves me with a fairly difficult to close bolt. I called Redding to ask why their die would not resize my brass, and they said, that because of my chamber I need to turn my die down incrementally until the brass fit in with just a very small amount of resistance. I have done that and now the brass fits in there perfectly. I haven't yet had the chance to make it to the range, but thanks so much for everyone's help. Although I have reloaded for years, and in fact I doubt I have ever fired a factory round, (except in something like a 30-30). All this longe range and precision reloading is fairly new to me. Thanks!

 

[MMERSS]

It is interesting you ask about lighting, because on the day I chronographed it was kind of hazy/foggy with intermittent blue sky. I don't even know if this could make a difference.

Lighting can make a difference.

Enjoy this article
Chronograph Accuracy Tips – 15 Practical Tips to Increase Accuracy & Reliability | PrecisionRifleBlog.com

 

[Bart B]

Here's some good info on making a 6.5x.284 drive pins; they're tinier than tacks.

6.5-284 Cartridge Guide

Big velocity spreads can be caused by two things.

One's the firing pin spring; it needs to be at or a bit above factory specs. If yours is more than 3 years old, I would replace it.

The other's two fold and has to do with the primer not being dented enough. One cause is the firing pin's not sticking out of the bolt face enough to dent the primer cup deep enough for consistant detonation of the primer pellet. .060" is minimum and .065" about maximum. The other is your fired cases, if full length sized, have their shoulders set back too far. .002" bump back from fired position's enough.

 

[Greyfox]

I did a lot a work on perfecting ES in my 6.5x284's. The basic case prep and components are 140 Hunting Berger VLDs, or 142 JLK's which perform identically, the only difference a .612 vs .640 BC. I use Lapua brass and neck size with.292 Reddng bushing dyes, neck sized, head-spaced .002"under fired brass length. I'll body size and anneal every 4-5 firings. Runout is less than .002". For all components, I always use the same lot. For a few rifles I have set up, the above is my base. Once I get good accuracy, .25-.5MOA at 200 yards, I then chronograph for velocity and ES. My goal with the 140's is to get velocity in excess of 2950 and ES of under 10-12 FPS for 5 shots and repaeatable over different days.
For optimizing ES and velocity the single biggest variable is getting good chronograph data. I have used several brands including an Oehler 35 and set-up and lighting have to be dead nuts repeatable. For me this has been the biggest source of false ES.Note: (IMO, greatest contribution to getting readings easily and accurately is the recently introduced Magnetospeed. Since using this I have elimated this variable for me!)
With my rifles I have found ES to be effected by:
Primer type and brand.
Charge weight
Seating depth. There are different accuracy and ES nodes with VLD's. I have found good accuracy "and" ES more often at .050-.1" off the lands.
Powder Type
I only change one at a time. My go to powder for hunting loads is Retumbo. I'll start with charge weight for velocity, ES, and accuracy with .075" off the lands, and try to optimize first varying seating depth. If this doesnt get me there, I'll try different primers. I start with Fed215's, but will try CCI mag, Fed210's is a last choice, but you will loose 50 fPS velocity. I had one rifle that needed a powder change to H4831sc to get then results I wanted. This is the process that I follow, but will have to say that the it's not as bad as it looks. The last two rifles I worked with got there with 57.5 gr. of Retumbo, 215's, and a seating depth of .075" off the lands. 2970FPS, ES 11, sub .5MOA. I only played with seating depth and charge. I think this is a good starting point.

 

[Bart B]

One other thing that will add to the muzzle velocity spread is how consistant the shooter holds the rifle against his shoulder. Folks who've compared muzzle velocities with a shoulder fired rifle and a machine rested one see a greater spread when the rifle's hand held. Us humans don't hold rifles against us exactly the same from shot to shot.

 

[Mikecr]

I agree with folks about the chronograph issues. It wasn't until I went to 20' screen spacing with an Oehler, that I could see ES really well.
Other things
Weighed charges of course
Load development/components/seating depth/Load density
Consistent neck tension
Hold, as Bart mentioned
Consistent sizing as measured
Consistent fouling(oil burnt out/removed) and shot timing/bore temps
Matched case H20 capacities(as properly measured)
Firing pin/spring/trigger/action timing settings
Reduced chamber end clearances(trim length)
Reduced neck and throat clearances
Consistent shell loading with low load density loads
Lower muzzle pressures(long enough barrel for load)
Faster powders + higher pressures + good load density + low muzzle pressures (ala underbore BR cartridges)

 

[jkupper]

Not to hijack the thread, but what is considered to be a low value for the deviation?

 

[Bart B]

Regarding shot timing and bore temperature . . .

Shoot 3 or 4 rounds to get the barrel fouled and warm.

Fire each shot at the same time interval. 30 seconds, 1, 2 or 3 minutes apart. Whatever you want.

Chamber each round 10 or 15 seconds then fire it. Each round needs the same chamber time else its powder will be at a different temperature for each shot; velocity will vary with powder temp. Hotter powder produces faster velocities.

And use the "SAAMI Twist" which is a repeatable way of taking a cartridge from its box, turning it end over end, then holding it horizontal as its loaded in the rifle. Then the rifle is handled the same way each time a round's fired. All this does is repeatably distribute the powder in the case for more uniform ignition and burning. Page 95 and 96 in the following SAAMI link has the instructions.

http://www.saami.org/specifications_and_information/publications/download/206.pdf

Great values of standard deviation of muzzle velocity is under 10 fps. That said, many a rifle will shoot well under 1 MOA at 1000 yards with standard deviations in the 20's. That's caused by how the barrel whips in the vertical axis when shot. Slower bullets leave when the muzzle axis points at a higher angle while its swinging up; faster ones leave sooner while that axis is at a lower angle on its up swing. This often happens and it sometimes makes long range accuracy better than short range accuracy.

 

[Greyfox]

Not to hijack the thread, but what is considered to be a low value for the deviation?

It really depends on what your range requirement and target size is. For example my 6.5x284 at 1000 yards, 2964FOS with a Berger 140VLD with an ES of 20FPS will calculate to about 4" vertical variability induced by this ES. At an ES of 10FPS it's 2". This can be calculated using a ballistic calculator by plugging in the velocity spread. While I generally try to get my ES around 10FPS for long range work for medium size game out to 1000 yards, 10-20 FPS will do. My current 6.5x284 load has an ES of 11FPS. The best ES out of my hunting rifles is 8FPS with my 300WM.

 

[MontanaRifleman]

Thanks for al lthe responses on my last post. I have a newly built 6.5 284 Norma built on a tika action with a 28 inch Burx barrel giving me some nice little 1/4 clusters at 100 yards. I have tried two powders, reloader 22 and H4831SC. Everybody says to go with Hodgeson so that is what I am going to do. So here is what my velocities are now:
2980
2957
2980
2985
2963
2981
2964
Basically I only hae a 100 yard range to work with and I only have CCI bench rest primers. I am shooting Berger 140 grain VLD's at 10.000ths down from full jam. I could probably move the bullet in alittle bit, but the group starts to open up if I move it out. I am just not sure what the practices are to get this extreme spread down. ANy advice would be greatly helpful...forgot to mention I am loading 52.8 grains of Hodgeson.

Put 2 off the shelf generic chronys back to back some day and you will never trust one again, not to mention what light conditions do to readings.

If you want accurate velocities, get a magnetospeed. Otherwise trust your groups down range. A tight ES doesn't necessarily mean a precision load. Last, SD means next to nothing unless you have large sample sizes.

 

[jkupper]

It really depends on what your range requirement and target size is. For example my 6.5x284 at 1000 yards, 2964FOS with a Berger 140VLD with an ES of 20FPS will calculate to about 4" vertical variability induced by this ES. At an ES of 10FPS it's 2". This can be calculated using a ballistic calculator by plugging in the velocity spread. While I generally try to get my ES around 10FPS for long range work for medium size game out to 1000 yards, 10-20 FPS will do. My current 6.5x284 load has an ES of 11FPS. The best ES out of my hunting rifles is 8FPS with my 300WM.

Thanks for the answer. I am currently getting an 18 fps variance with my 7 RM, and I though that was pretty good. I think I can clean it up a bit more, but it works for what I am currently doing. This thread really helps!